1. Field of the Invention
The present invention relates to disc defect detection in data reproduction systems. More specifically, a method and an apparatus for accurately generating disc defect signals during data recording and/or data reproduction in an optical disc system is disclosed.
2. Description of the Prior Art
In data reproduction systems, such as a CD-RW drive and a DVD-RW drive, it is important to know which areas of a disc are unusable due to defects. In an information recording system that supports defect management, a defect circuit must be able to generate accurate defect signals for the correct duration of the defect during data recording and/or data reproduction.
It is well known that an area of a disc containing a defect reflects the light differently than other areas of the same disc. A conventional defect detection system uses this variation in a reflected light intensity signal to determine the presence or absence of a disc defect. The general prior art method of determining disc defects involves comparing the currently reflected light intensity signal with an average of the same reflected light intensity signal. If the difference between the reflected light intensity signal and an averaged reflected light intensity signal value over a predetermined time period is larger than a preset threshold, a defect signal is generated.
FIG. 1 is a defect detection circuit 20 according to a prior art. The defect detection circuit 20 comprises a low-pass filter 24, a subtractor 28, two comparators 22a and 22b, and an OR operator 26. A reflected light intensity signal (SBAD) is split with one portion sent through the low-pass filter 24 creating a new, low-frequency signal (SBAD_LPF). The subtractor 28 subtracts the SBAD_LPF from the original SBAD and transmits the result to the two comparators 22a and 22b. The comparator 22a outputs “true” only if the result is larger than a preset positive threshold (DFTH_P). Similarly, the comparator 22b outputs “true” only if the result is smaller than a preset negative threshold (DFTH_N). If the OR operator 26 receives a “true” value from either of the comparators 22a or 22b, a disc defect signal is generated.
FIG. 2 shows the various signals throughout the detection process. FIG. 2 illustrates a defect signal generated by the comparator 22b based on a result smaller than the preset negative threshold (DFTH_N). However, it should be noted that a signal based on exceeding the DFTH_P would function similarly. In practice, the use of one positive threshold (DFTH_P) and one negative threshold (DFTH_N) commonly, but not necessarily, functions the same as taking the absolute value of the result from the subtractor 28 and having only one threshold.
The left side of FIG. 2 shows signals generated over an area of the disc containing no defects. Therefore, the incoming SBAD and the low-passed SBAD_LPF are approximately equal. When a disc defect occurs, the intensity of the SBAD changes far faster than the SBAD_LPF, creating a divergence that quickly exceeds the preset negative threshold (DFTH_N), causing a defect signal to be generated. When the area of the disc containing the defect has passed and another area of the disc is again reflecting the sub-beams, the SBAD returns to a normal value. Now, the difference between the SBAD and the SBAD_LPF is again less than the threshold, resulting in a defect signal no longer being generated.
The main benefit of the above defect detection method is that a disc defect can be also detected during recording with the SBAD signal by sampling the sub-beams of the optical pickup in land recording. However, the above defect detection method fails and generates inaccurate defect signals during a long defect as shown in FIG. 3. The main reason is that the average of the SBAD gradually shadows the SBAD when the defect period is relatively longer than the time constant of the low-passed filter. The defect signal shown in FIG. 3 starts at point T1 and erroneously ends at point T2 because of the difference between the SBAD and the averaged SBAD_LPF is smaller than the preset threshold. Additionally, an erroneous defect signal is generated from point T3 to point T4 during which the SBAD_LPF cannot return to the normal SBAD level rapidly enough and the difference between SBAD and SBAD_LPF is overly large, again triggering the defect signal. The prior art generated defect signal can provide false information about the length of the defect, which may cause failure in disc defect management.